The present invention relates to position measuring systems, and more particularly concerns laser position measurement of a moving machine tool member.
In a wide variety of machine tool systems a tool is moved to a preselected location at which it is to perform an operation such as drilling, routing, and placement of parts, among others. For such operations it is essential to know the location of the tool so that the hole can be drilled in a particular position or a part can be placed at desired location, etc. Preferably, for precision positioning of a tool, a closed loop system is employed in which actual position of the tool is measured and compared with a commanded position to derive an error or difference signal used to drive the tool. Many different types of position measuring systems have been employed but all suffer from lack of sufficient accuracy for precise positioning systems or excessive cost.
Among the most precise measuring systems are those employing a laser for making interference measurements. These systems include a laser generating unit, an interferometer and a receiver. A typical system also includes a separate corner reflector and may have a display unit for displaying measurement numbers. However, such laser measuring systems for machine tool operations are severely limited by the fact that the laser interferometer can measure only a distance or change of distance in a fixed direction between the laser beam and the reflector. Because tool positioning must be carried out while the tool is in motion, the typical laser interferometer measuring system cannot be employed for directly measuring position of a moving tool. As the tool moves, its corner reflector moves with it, but where the laser beam is fixed and has no arrangement to keep the beam directed to the moving corner reflector on the moving tool, laser measurement cannot be used.
In most machine tool positioning systems the tool support structure or carriage is constrained by guideways generally aligned in mutually orthogonal X and Y orientations so that the carriage and tool may move to any given X,Y position within an X,Y plane by a combination of motions along the axes. Lengths of the X axis motion and Y axis motion are measured to identify the resulting tool position. However, accuracy of such systems is severely limited by difficulties in manufacturing precision guideways. As accuracy requirements are increased, cost of the manufacture of the precision guideways increases rapidly. Such guideways are required to be perfectly straight, without small bends, and must be precisely aligned in the selected direction. Manufacture is a costly procedure, and therefore precision systems employing guided ways are very expensive. Such guideways do lend themselves to accurate distance measurement in a fixed direction, since a laser may be employed to measure the distance traveled along X and Y axes. However, laser measurement is not available for measurement of systems that do not travel in predetermined directions on precision guideways, since the laser system can only measure distance in a predetermined direction, and laser machine tool sensors have no tracking or target finding ability.
Accordingly, it is an object of the present invention to provide a position measuring system that avoids or eliminates above mentioned problems.